Ah, thank you! I don't know why I was trying to use "_". Totally unnecessary. Now that you have helped me to fix most of these basic problems, I am back to the original problem of replacing the hash keys with the hash values. The hash is created with:

use strict;
use warnings;
open my $fh, '<', 'hash_test.txt' 
    or die "Cant open file $!";

LINE:
while (my $line = <$fh>) 
    {
        my ($key, $value) = split /\s/, $line;
        next LINE if not $key;
        $hash{$key} = $value;
        chomp (%hash);
        while ( my( $key, $value ) = each %hash )          
    }

close $fh;
[download]

Then the hash keys are searched for (and replaced with their associated values if found) in the input file using

open FH, "<infile.txt" or die $!;
open OUT, ">outfile.txt";

 while (<FH>)
          {
          if (/(\S+):(\S+).*\n/)
               {
               $var1 = "$1";
               $var2 = "$2";
               print OUT "$hash{$var1} $var2\n";
               }
            elsif (/(.*)\n/)
                {
                print OUT "$1\n";
               }
            }
     close FH;
[download]

This results in the error

Use of uninitialized value within %hash in concatenation (.) or string at test_perl.pl line 41, <FH> line 5

UPDATE I think the problem was that I had initialized my %hash inside a while loop. I moved it up to the beginning of the program, and it worked!