sub encode {
    my ($text, @alphabet) = @_;
    local $" = '';
    eval "\$text =~ tr/a-z/@alphabet/";
    $_ = uc for @alphabet;
    eval "\$text =~ tr/A-Z/@alphabet/";
    $text
}

sub decode {
    my ($text, @alphabet) = @_;
    local $" = '';
    eval "\$text =~ tr/@alphabet/a-z/";
    $_ = uc for @alphabet;
    eval "\$text =~ tr/@alphabet/A-Z/";
    $text
}
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Instead of running eval twice per subroutine you could do it like this:

sub encode {
    my ($text, @alphabet) = @_;
    local $" = '';
    eval "\$text =~ tr/a-zA-Z/\L@alphabet\U@alphabet\E/";
    $text
}

sub decode {
    my ($text, @alphabet) = @_;
    local $" = '';
    eval "\$text =~ tr/\L@alphabet\U@alphabet\E/a-zA-Z/";
    $text
}
[download]