my %trans = (
   a => "alpha",
   b => "beta",
   ...
   z => "zulu"
);

my $line = 'AbZaBz';

foreach (split //, $line) {
    if (exists $trans{lc($_)}) {
        print (($_ ne lc($_)) ? uc($trans{lc($_)}) : $trans{$_});
    } else {
        print $_;
    }
    print ' ';
}

__OUTPUT__
ALPHA beta ZULU alpha BETA zulu