Almost always true in practice. But there is more overhead in a hash than in an array, and that's magnified if you already have an array and have to duplicate it as a hash. If you're going to check more than one value against the list, as in the FAQ "How do I search file2 for all the values in file1?" then a hash is almost certainly the best solution. But if you only have to check one or two values, you may be better off sticking with grep:

 % cat 986735.pl
#!/usr/bin/env perl
use Modern::Perl;
use Benchmark qw(:all);

my @words = split /\s+/, `cat bigfile`; #8.5MB file
say scalar @words, " words in bigfile"; #1.3M words
my $match = 'professional'; # appears 16 times scattered through bigfi
+le

cmpthese( 10, {
        'hash it' => \&hashit,
        'grep it' => \&grepit,
        'first it' => \&firstit,
});

sub hashit {
    my %h;
    @h{@words} = ();
    my $exists = exists $h{$match};
}

sub grepit {
    my $exists = grep { $_ eq $match } @words;
}

sub firstit {
    use List::Util 'first';
    my $exists = first { $_ eq $match } @words;
}
 % perl 986735.pl
1293687 words in bigfile
           Rate  hash it  grep it first it
hash it  3.33/s       --     -43%     -89%
grep it  5.80/s      74%       --     -81%
first it 29.8/s     795%     413%       --
[download]

So if I only need to search the list once, grep wins over a hash. For multiple searches, a hash comes out ahead. List::Util's first() routine splits the difference a little; with my dataset it beats the hash for up to about 8 searches, but I assume that would vary greatly depending on how early in the array the match is made. It'd take more testing with matches found earlier/later/never in the array to form a good comparison there.

My simple conclusion would be: always build a hash unless you know your program will only search it once; then use grep or List::Util::first. Also, if I'm pulling data in from somewhere (like a file) and I know I'm going to be searching it this way, I put it straight into a hash from the start.