$ cat script.pl
use warnings;                                                                                                                                                                                                                                
use strict;                                                                                                                                                                                                                                  
                                                                                                                                                                                                                                             
my $a = "file_abc_123.txt";                                                                                                                                                                                                                  
my $b = 123;                                                                                                                                                                                                                                 
my $c = 100;                                                                                                                                                                                                                                 
                                                                                                                                                                                                                                             
if ( $a =~ m/$b/ ) {                                                                                                                                                                                                                         
        $a =~ s/$b/$c/;                                                                                                                                                                                                                      
}                                                                                                                                                                                                                                            
                                                                                                                                                                                                                                             
printf "%s\n", $a;                                                                                                                                                                                                                           
$ perl script.pl                                                                                                                                                                                                           
file_abc_100.txt