1. When I changed the $_ to a numerical value inside the regex instead:

if ($test =~m#(??{substr($test, 0, 8)})#) {  }
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2. When I set the string $test to a something numerical (12,444.343,etc.) and again it will work without warning.

3. I tried setting $_ to a number outside and again it failed.

use strict;
use warnings;

my $test = "fee fi fo fee";
$_++;

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

my $test = "fee fi fo free";

if ($test =~ m!((??{ print "$_\n";substr($test,0,$_) }))!) {
};
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fee fi fo free
Argument "fee fi fo free" isn't numeric in substr at (re_eval 1) line 
+1.
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As one who was puzzled by this in cb, I was happy to see this sensible explanation. I really thought you'd found the answer, but this one-liner seems to say not: perl -e'($foo="bar") =~ m/($_)/;print $1,$/'

Update: ++ Enlil and BrowserUK for figuring this out! It's not m// but (??{}) that does it.

That's a fact worth knowing, and props for curiosity over efficiency! A modest application:

sub isPalindrome {
    $_[0] =~ /(??{quotemeta scalar reverse $_})/i
}

sub isAllUpper {
    $_[0] =~ /(??{uc})/
}
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After Compline,
Zaxo

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.

I'm slightly confused by your goal? I assume that this is intended to be used inside a loop with $_ as the loop var, so when I saw Enlil's conclusions that m// is messing with $_ I thought why not avoid m// and use index and came up with this.

if( 1+index($test,substr($test,0,$_)) ){ # Do stuff }
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looked like it would do the same test as your 2 steps, but then I realised that what this translates to is

 if( $test contains a substring of $test )...

which by definition, will always be true!

Did I miss something?

Without trying to sound facetious... Why would you want to do a thing like that?

I can't figure out the purpose. It seems to me as if you are taking a leading substring from a string, and then seeing if you match the substring in the string. Which you always will, unless the substring happens to contain meta-characters which will be interpreted differently by the regexp engine.

So if that is the case, why don't you just see whether

  substr($test, 0, $_) eq quotemeta(substr($test), 0, $_))
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Of course, I may be overlooking something obvious, so feel free to clue me in.

#$url = substr($test, 0, $_);
#if ($test =~ /$url/ {
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What happens if you don't use $_ as this may be getting corrupted durring the evaluation

Hope it helps
UnderMine