Are all 'special variables' ($&, $N (N is integer)) expanded in that way too?

my $var = '[ab]';
my $string = '12a3434';
print "match" if $string =~ /$var/;
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$temp =~ m/(\[0-9\])blah$1/;
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I think you meant

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

Update: added demo code.