my %word;
{
    open my $fh, '<', '/path/to/wordlist.txt' or die $!;
    chomp( my @words = <$fh>);
    close $fh or die $!;
    @word{@words} = ();
}
# ... then when it's time to look up a word,
    if (exists $word{$foo}) {
        # ...
    }