The m// or s/// work more like double quotes, and you can't include constants into double quotes this way, either.

print "THIS APOSTROPHE MATCHES" =~ /APOSTROPHE/;  # 1
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There are tricks how to do it, though:

use constant APOSTROPHE => chr 39;
print chr(39) =~ /${ \ APOSTROPHE }/;  # 1
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Update: s/(?<=t)(?=icks)/r/