Juerd explained how to do this with eval, but there is another way. I saw this illustrated in a node a couple of months back, which of course I can no longer find, so I'll try to reproduce the concept. Basically the idea is that we build an array of the $1 .. $n values then do a second substitution on that. The array is required because the substitution clobbers the variables: 
sub dyn_replace {
  my ($replace) = @_;
  my @groups;
  {
    no strict 'refs';
    $groups[$_] = $$_ for 1 .. $#-;      # the size of @- tells us the
+ number of capturing groups
  }
  $replace =~ s/\$(\d+)/$groups[$1]/g;
  return $replace;
}

[download]
and we use it as follows 
my $pat = qr/foo(\d+)bar(\d+)/;
my $rep = 'x$1y$2';
my $str = 'foo36bar47';
$str =~ s/$pat/dyn_replace($rep)/e;
print "$str\n";

[download]
and it prints "x36y47", just as desired.

In reply to Re: s/// in a sub : passing expressions by Errto
in thread s/// in a sub : passing expressions by ady

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