Simpler:

use v5.14;

sub encode_decode {
    shift =~ tr/A-Za-z/ZYXWVUTSRQPONMLKJIHGFEDCBAzyxwvutsrqponmlkjihgf
+edcba/r;
}

my $enc = encode_decode("this is a test");
my $dec = encode_decode($enc);
say "Enc: ", $enc;
say "Dec: ", $dec;

__END__
Enc: gsrh rh z gvhg
Dec: this is a test
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