Is this homework? The substitution operator (s///) is documented in perldoc perlop. Look for the option that allows you to specify an executable expression as the replacement string.

Also, there is no need to look for a match before doing the replacement. Just do the substitution.

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