Based on your code sample, might I suggest a hash lookup table, eg:

# Untested
my @filesize_names = qw(kilobyte megabyte gigabyte terabyte petabyte e
+xabyte zettabyte yottabyte);
my %short_sizes = map { m/^(\w)/; $1 . 'b' => $_ } @filesize_names;
my $word = 'kb';
if ( exists $short_sizes{lc $word} ) {
    say $short_sizes{lc $word};
}
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