use Magic;
tilt() if ($str =~ m/$Magic::selfish/ 
    && ${str}abc !~ m/$Magic::selfish/);
tilt() if ($Magic::selfish =~ m/$str/ 
    && ${Magic::selfish}abc !~ m/$str/);
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Matching is a one-to-many operation. More succintly:

die "42" if ${Magic::selfish}z !~ m/$Magic::selfish/;
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You're tilting at a windmill.

After Compline,
Zaxo