... any $string can be used as a valid regex like in m/$string/ ...

... and you don't even need the  m// or  qr// incantation. The  =~ binding operator can do the trick: 

>perl -wMstrict -le
"my $s = 'abc';
 ;;
 print 'match' if 'xabcx' =~ $s;
 print 'match' if 'xabcx' =~ 'abc';
"
match
match

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In reply to Re^2: Detecting if a string is a regular expression by AnomalousMonk
in thread Detecting if a string is a regular expression by smontsar

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