#!/usr/bin/perl

use strict;

my $old_string = "the dog chased my cat.";
my ($new_string, $found_word);
my %dict;

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

chop( $old_string );
my @words = split( " ", $old_string);

foreach my $i ( @words ) {
    foreach my $j ( keys %dict ) {
        if ( $j eq $i ) {
            $new_string = $new_string . "$dict{$j} ";
            $found_word++;
        }
    }
    $new_string = $new_string . "$i " if ($found_word == 0);
    $found_word = 0;
}

chop( $new_string );
print "$new_string.";

__DATA__
1 dog
2 cat
3 chased
4 the