$ perl -MO=Deparse fonality.pl
s/\n(?:SEEKING)?/\nPERLqny~%|fsyx%~tz&/ ? $^X =~ /\w+$/ : '???';
s/$&/Ktsf/i;
s/./chr ord($&) - 5;/eg;
$\ = $/;
'???' unless print $_;
fonality.pl syntax OK
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Let's pick it apart.

s;
(?:SEEKING)?;
PERLqny~%|fsyx%~tz&;
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This is does nothing. It's a replacement on $_, which has the empty string in it. The replacement does nothing because it doesn't match the empty string because the empty string doesn't have a \n in it. It's worth knowing that it doesn't match because of what comes next:

?$^X=~m.\w+$.:DEVELOPERS;
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The question mark at the front uses the replacement (s///) as a condition, so the stuff after the question mark and leading up to the colon is not executed. DEVELOPERS is a bareword, which in this case will be interpreted as a string, which does nothing.

s"$&"Ktsf"i;
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This is another replacement on $_. At this point, $& (aka $MATCH, if you use English, see perlvar) has nothing in it, and so does $_, so this matches. After the replacement, $_ eq 'Ktsf'.

s^.^chr ord($&)-5^eg
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Here's another replacement. In this case, we're replacing every single character in $_ with the character that's five characters back on the ASCII chart. For example, chr ord('K') - 5 is 'F'. The /g flag is what makes this replace every character, and the /e flag is what makes it interpret the replacement expression as code instead of text. See chr and ord. After this, $_ eq 'Fona'

$\=$/
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See perlvar for these. The output record separator is being set to the same value as the input record separator, which is still at its default, newline ("\n"). That is, $\="\n". This means that when you print something, it will have newline appended for you. That's convenient for the next part.

print||" ;) "
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This prints the contents of $_, with $\ (newline) appended. Since print returns true on success, stuff after the || is not executed. Even if it were, it's a string, so it does nothing.