Works for me:

use strict;
use warnings;
my $string = "hello = 1234ab;cat;dog;4892";

my $semiColon=2;
if ( $string =~ /hello.*=(\s*.*;\s*){$semiColon}(\s*.*;\s*).*/i ) {
  my $substring = $2;
  print "$substring\n";
}
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I would recommend that you at least modify your dot-stars to be .*? (non-greedy). And get rid of the trailing one. That one's useless.

/hello.*?=(\s*.*?;\s*){$semiColon}(\s*.*?;\s*)/i
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