The caller function might be you best bet. It can be used to examine the call stack. Nested evals have '(eval)' as the subroutine name:

#!/usr/bin/perl

use strict;
use warnings;

sub eval_depth {

  my $eval_depth = 0;
  my $frame = 0;

  while (my @caller_info = caller($frame++)) {

      if ($caller_info[3] eq '(eval)') {
        $eval_depth++;
      }
  }

  return $eval_depth;
}

print "no evals: ".eval_depth()."\n";
eval 'print "one eval :".eval_depth()."\n"';
eval q{ eval 'print "two evals:".eval_depth()."\n"'};
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