#!/usr/bin/perl -w
use strict;

open WORDS, "< words" or die "cant' open words";
my (@words, @sentences);
while (<WORDS>) {
    chomp;
    push @words, [$_, qr/\b$_\b/];
}
close WORDS;
open SENTENCES, "< sentences" or die "cant' open sentences";
while (<SENTENCES>) {
    my $printed = 0;
    for my $word (@words) {
        if (/$word->[1]/) {
            print $_ unless $printed++;
            print "\t", $word->[0];
        }
    }
    print "\n" if $printed;
}
close SENTENCES;
__END__

program's output:
-------------------------------
I am the first
        first
I always wanted to be the first
        first
I never liked to be second
        second
I second your request
        second
Better second than third
        second  third