Use a hash, that way you only need to take one pass over the file:

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

my %terms;

for (<DATA>) {
    chomp;
    $terms{$_}++;
}

for my $key (keys %terms) {
    print "$key appeared $terms{$key}\n";
}

__DATA__
one
two
three
one
five
two
ten
one
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Update: Well it's very pleasing to find that no less a luminary than Brother choroba has arrived at the exact same answer, albeit of course much faster!