Shell Programming and Scripting

use warnings;
use strict;

die qq[Usage: perl $0 <input-file>\n] unless @ARGV == 1;

my ($shift_port, $port3, @lines, @ports);

while ( <> ) { 
    chomp;

    if ( m/\A\s*(?i:shift_port)\s*=\s*(\S+)/ ) { 
        $shift_port = $1; 
    }   

    if ( m/\A\s*(?i:port3)\s*=\s*(\S+)/ ) { 
        $port3 = $1; 
    }   

    if ( $shift_port ) { 
        push @lines, $_; 
    }   

    if ( $shift_port && $port3 ) { 
        push @ports, [ $shift_port, $port3 ];
        undef $port3;
    }   


    if ( @ports == 2 ) { 
        if ( index( $ports[0][0], qq[P0] ) == 0  &&  
             index( $ports[0][1], qq[0] ) == 0  &&  
             index( $ports[1][0], qq[P0] ) == 0 &&  
             index( $ports[1][1], qq[0] ) == 0   
        ) { 
            $lines[ 0 ] =~ s/P0/M0/;
            $lines[ $#lines - 1 ] =~ s/P0/00/;
            printf qq[%s\n], join qq[\n], splice @lines;
            $shift_port = undef;
            $port3 = undef;
            @ports = (); 
        }   
        else {
            printf qq[%s\n], join qq[\n], splice @lines, 0, @lines - 2;
            shift @ports;
        }   

        next;
    }   

    unless ( $shift_port || $port3 ) { 
        printf qq[%s\n], $_; 
    }   

    if ( eof ) { 
        printf qq[%s\n], join qq[\n], splice @lines;
    }   
}

perl script.pl infile