It doesn't return 1 for success. It returns the number of matches (or substitutions for s/// or tr///). If you're not using the g option, it stops on the first match and returns 1. I'm not really up on perl internals but I would guess undef is represented as a null pointer whereas a zero would require creation of an SV.

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print map { my ($m)=1<<hex($_)&11?' ':'';
$m.=substr('AHJPacehklnorstu',hex($_),1) }
split //,'2fde0abe76c36c914586c';
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