I started playing with the perl debugger and I still think I am right though I do not quite know the reasoning behind it. The debugging went as follows ( I am omitting much, but leaving the important parts). First the program: 
use strict;
use warnings;

my $test = "fee fi fo free";

if ($test =~ m!((??{ substr($test,0,$_) }))!) {};

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Now the debugging stuff:
 DB<1> V main          # $_ never appears after this command
 DB<2> s
main::(bah4.pl:6):      if ($test =~ m!((??{ substr($test,0,$_) }))!) 
+{};
 DB<2> V main          # $_ never appears after this command
 DB<3> s
main::((reeval 2)[bah4.pl:6]:1):
 DB<3> V main    #At this point again variables in main are 
                 #listed there is a line: 
                 #  $_ = 'fee fi fo free'
  DB<4> s
Argument "fee fi fo free" isn't numeric in substr at (reeval 2)[bah4.p
+l:6] line 1.

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and then a bunch of error stuff. I ran the one liner through the debugger and $_ never gets set, but as I mentioned before I do not know why the difference, perchance one of the many wiser monks than I might enlighten us at this point.

-enlil

*Note: I have not used the debugger all the much, so if I managed to confuse myself and thereby gave out misinformation let me know.

In reply to Re: Re: Re: Troubles with m!(??{substr(...)})! by Enlil
in thread Troubles with m!(??{substr(...)})! by jens

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