You're use of $date is unlikely to be what you intended. Assuming the second my $date; isn't supposed to be there, we're left with:

sub num_month{
  my $date = shift;
  ...
  foreach ...{
    my $date = ...  # <----this is a new $date scalar, which does not 
+exist outside of this foreach loop
  }

  return $date;  #<----this is the $date from before the foreach loop 
+-- ie, exactly what was passed to the sub
}
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You can see the behavior by running this short script:

use strict;
use warnings;

my $test = &GO(1);
print $test;

sub GO{
  my $val = shift;
  foreach (0..10){
    my $val += $_;
    print $val . "\n";
  }
  return $val;
}

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