The sprintf() method: my $hex_number = sprintf "%lx", $number; That's the best way I know. Another option would be:

my %table = (
    'a' => '10',
    'b' => '11',
    'c' => '12',
    'd' => '13',
    'e' => '14',
    'f' => '15',
);

my @numbers = ('a0', '11', 'ff', '9084fe', '10');

foreach my $trial (@numbers) {
    my @digits = split //, $trial;
    my $result = 0;
    foreach my $digit (@digits) {
        $result *= 16;
        $result += 
            $digit =~ /\d/ ? 
                $digit :
                $table{lc($digit)};
    }
    print "$trial is actually $result\n";
}
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I wouldn't do it that way very often, though. The internal conversion routine is much faster.

modified on 9 April 'cuz btrott is right in his comment, that sneaky japh.