#!/usr/bin/perl
use strict; 
use warnings;

my %dict;
while ( <DATA> ) {
    my ( $key, $val ) = /^(\d+)\s+(\S+)/;
    $dict{ $val } = $key;
    }

my $regex = '\b' . join('\b|\b', map "\Q$_\E", sort {length $b <=> length $a} keys %dict) . '\b';

open my $fh, '<', 'junk.txt' or die "Open failed: $!";

while (defined(my $line = <$fh>)) {
    $line =~ s/($regex)/$dict{lc($1)}/ieg;
    print $line;
}
__DATA__
1 eat
2 habit
3 boy
4 man-kind
5 man
6 kind

##</code><code>##

The boy has a habit of eating kind of like a man.  Man-kind, as a group, does
not wear habits.  Kind of you to watch what you eat.

##</code><code>##

The 3 has a 2 of eating 6 of like a 5.  4, as a group, does
not wear habits.  6 of you to watch what you 1.