Quoth perlre (see section on Capture buffers):

The numbered match variables ($1, $2, $3, etc.) and the related punctuation set ($+, $&, $`, $', and $^N) are all dynamically scoped until the end of the enclosing block or until the next successful match, whichever comes first. (See "Compound Statements" in perlsyn.)

Update: Try this with and without the  { } block enclosing the substitution statement (Update: changed example code to try to clarify the point a bit): 

>perl -wMstrict -le
"my $line = 'One two three four five';
 if ($line =~ m/(one)\s(two)\s(three)\s(four)\s(five)/i){
   my $neo;
   { ($neo = $1) =~ s{ (o)(ne) }{$2$1}xmsi; }
   print qq{neo '$neo'};
   print qq{1: $1};  print qq{2: $2};  print qq{3: $3};
   print qq{4: $4};  print qq{5: $5};
   }
"
neo 'neO'
1: One
2: two
3: three
4: four
5: five

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In reply to Re: Regexp and substitution question by AnomalousMonk
in thread Regexp and substitution question by bdalzell

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