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';
[download]

You could also look at using $INPLACE_EDIT ($^I) to have perl do the backup and rename for you.