You can't use %INC.

$ perl -le'no strict;      print $INC{ "strict.pm" } ?1:0'
1   # False positive

$ perl -le'{ use strict; } print $INC{ "strict.pm" } ?1:0'
1   # False positive

$ perl -le'use JSON;       print $INC{ "strict.pm" } ?1:0'
1   # False positive

$ perl -le'use v5.20;      print $INC{ "strict.pm" } ?1:0'
0   # False negative
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You can use ( caller(0) )[8] in a sub.

$ perl -le'
   sub f { ( ( caller(0) )[8] & 0x602 ) == 0x602 }
   print f ?1:0;
'
0

$ perl -le'
   sub f { ( ( caller(0) )[8] & 0x602 ) == 0x602 }
   use strict;
   print f ?1:0;
'
1

$ perl -le'
   sub f { ( ( caller(0) )[8] & 0x602 ) == 0x602 }
   use v5.20;
   print f ?1:0;
'
1
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0x2 is strict refs, 0x200 is strict subs, and 0x400 is strict vars.