I would start with open FH, "<file";

No, you should not. You should always test the return value of open. Furthermore, I wouldn't use second class variables if I can avoid it. Finally, you may prefer 3-arg open over magic 2-arg open:

    open my $fh, "<", "file" or die "Failed to open 'file': $!";
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Abigail

Something like this (untested) code may be what your looking for.

#! perl -w
use strict;

open IN, '<', 'file' or die $!;
open OUT, '>', 'temp' or die $!;

while( <IN> ) {
    s[ (?<=\\|/) (.*?) (?=\\|/) ][ 
        (my$x=$1) =~ s{ }{_}g; 
        $x
    ]gex;
    print OUT;
}

close IN;
close OUT;
rename 'file', 'file.bak';
rename 'temp', 'file';
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You could also look at using $INPLACE_EDIT ($^I) to have perl do the backup and rename for you.