\1 is a back reference within the same regular expression which may be useful, among other things, to look for a repeated word. For example: 
  DB<1> $c = "The black dog and the black cat";

  DB<2> print "Matched" if $c =~ /(bl..k).+\1/;
Matched

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In a substitution expression, you should rather use $1.

Je suis Charlie.

In reply to Re: Substitution backreference woes by Laurent_R
in thread Substitution backreference woes by Anonymous Monk

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