$ cat script.pl
use strict;                                                                                                                                                                         
use warnings;                                                                                                                                                                       
                                                                                                                                                                                    
@ARGV || die "Usage: perl $0 file1 file2 ...\n";                                                                                                                                    
                                                                                                                                                                                    
while ( my $file = shift @ARGV ) {                                                                                                                                                  
    # Open files. In failure, warn the problem and read next file.                                                                                                                  
    open my $ifh, "<", $file or do { warn "Cannot open $file for reading: $!\n"; next };                                                                                            
    open my $ofh, ">", $file . ".out" or do { warn "Cannot open $file.out for writing: $!\n"; next };                                                                               
                                                                                                                                                                                    
    # Check type of file.                                                                                                                                                           
    while ( <$ifh> ) {                                                                                                                                                              
        # File 1.                                                                                                                                                                   
        if ( /^DTM/ ) {                                                                                                                                                             
            s/(DTM\+\d*:)\d{12}/$1 . ("'" x 12)/eg;                                                                                                                                 
        # File 2.                                                                                                                                                                   
        } elsif ( /^(?:EXIT|DISC)/ ) {                                                                                                                                              
            s/^(EXIT|DISC)\d{8}/$1 . ("'" x 8)/e;                                                                                                                                   
        # File 3.                                                                                                                                                                   
        } elsif ( /^HHDR/ ) {                                                                                                                                                       
            s/(HHDR\s+\d{6})\d{8}/$1 . ("'" x 8)/eg;                                                                                                                                
        }                                                                                                                                                                           
                                                                                                                                                                                    
        print $ofh $_;                                                                                                                                                              
    }                                                                                                                                                                               
                                                                                                                                                                                    
    close $ifh or warn "Error found closing $file: $!\n";                                                                                                                           
    close $ofh or warn "Error found closing $file.out: $!\n";                                                                                                                       
} 
$ perl script
Usage: perl script.pl file1 file2 ...
$ perl script.pl file1 file2 file3
(Output files will be appended with '.out' -> file1.out, file2.out and file3.out in this sample).