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

[download]

I think you meant 

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

[download]

in which case any special characters in the content of the backreference \1 would not be treated special. IOW, "[0-9]blah[0-9]" would match, but not "[0-9]blah6": 

#!/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";
    }
}

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prints 

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

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while, if you replace \1 with $1 in the above regex, it prints 

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

[download]

This is because $1 isn't defined here, thus the regex effectively becomes /(\[0-9\])blah/...

Update: added demo code.

In reply to Re^3: Regex error when [] occurs in file.. ($1 vs. \1) by almut
in thread Regex error when [] occurs in file.. by why_bird

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