It is just a personal preference, but i would make a little sub that would package away the addition, and do the input checking (i.e. remove any non-digits, warn if the number is -1000_000, or any other of the situations pointed out) and provide helpful errors if anything is bad.

use warnings;     ## or die!
use strict;       ## seriously - don't anger the gods
use diagnostics;  ## but if you do, make them tell you why (patiently)
+...

if (my $big_int = big_int('101_dalmations') ){
  ... it was OK so go ahead and play ...
}
else {
  ... no need for this bit, error messages already generated! ...
  ... maybe you want to do something else though? ...
}

### SUBS ###

sub big_int{
  my $input = shift;
  ## check for any source of errors
  if ( $input =~ /\D/ ){
    ## non-digits - argh!
    warn "$input contains non digits\n";
    return 0;
  }
  elsif (... something else ...){
    ...
  }

  return ($input + 1000_000);
}
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It just means that what you are actually trying to achieve (make a big int) remains clear, while also handling the errors nicely if they don't conform etc...

Just MHO though...