$ perl -we " /(a)$1/ "
Use of uninitialized value $1 in regexp compilation at -e line 1.
Use of uninitialized value $_ in pattern match (m//) at -e line 1.

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Basic debugging checklist, Tutorials: Use strict and warnings, Why you should use strict, diagnostics/Use strict warnings and diagnostics or die

In reply to Re: Difference between $1 and \1. by Anonymous Monk
in thread Difference between $1 and \1. by rbala

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