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$ perl -wE 'use re "/g"'
Unknown regular expression flag "g" at -e line 1.
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$ perl -wE 'my ($p, $f) = (A => "x"); my $re = qr{(?$f: $p )}; say $re'
(?^u:(?x: A ))
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$ perl -wE 'qr{(?g:)}'
Useless (?g) - use /g modifier in regex; marked by <-- HERE in m/(?g <-- HERE :)/ at -e line 1.