$temp =~ m/(\[0-9\])blah$1/;

##</code><code>##

$temp =~ m/(\[0-9\])blah\1/;

##</code><code>##

#!/usr/bin/perl

use strict;
use warnings;

for my $temp ("[0-9]blah[0-9]", "[0-9]blah6") {
    printf "%-15s ", $temp;
    if ($temp =~ /(\[0-9\])blah\1/) {
        print "matched\n";
    } else {
        print "didn't match\n";
    }
}

##</code><code>##

[0-9]blah[0-9]  matched
[0-9]blah6      didn't match

##</code><code>##

Use of uninitialized value in concatenation (.) or string at ./671663.pl line 8.
[0-9]blah[0-9]  matched
[0-9]blah6      matched