Sprad,
Since you didn't indicate you knew why you didn't receive a warning or an error, here is some code to illustrate the point:

#!/usr/bin/perl
use strict;
use warnings;
my $sub = 'foo';
{
    no strict 'refs';
    *{ $sub } = sub { return 'bar' };
}
print foo();
[download]

The definition of a sub can be made at runtime so it is hard to check at compile time that it has been defined. I am not saying that there isn't a way to say "ok - well except for that", but if there is a way I am sure someone else will reply. It is likely a scary hack involving do'ing the file, parsing for things that look like sub calls, and then using Universal can to see if main->can( 'sub' ).