perlre says: 
This is a "postponed" regular subexpression. It behaves in exactly the
+ same way as a (?{ code }) code block as described above, except that
+ its return value, rather than being assigned to $^R , is treated as 
+a pattern, compiled if it's a string (or used as-is if its a qr// obj
+ect), then matched as if it were inserted instead of this construct.

[download]
so what is treated as a pattern here? is it what "print $1" returns?

In reply to Re^6: Print A Sequence with Start codon and different Stop Codon by Anonymous Monk
in thread Print A Sequence with Start codon and different Stop Codon by PerlKc

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