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);
}