Just for clarity!

It's not the literal which makes the difference but - as already suspected in the OP - the constant folding.

#!/usr/bin/perl -l
 
sub gen_arr {
  my $inner =shift;
 
  print my $outer = \"foo$inner"; # still a literal but not constant a
+nymore
 
  return $outer;
}
 
gen_arr gen_arr;
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Use of uninitialized value $inner in concatenation (.) or string at /t
+mp/tst2.pl line 6.
SCALAR(0x912c760)
SCALAR(0x912c9d0)
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