I just can't figure out what's getting assigned to $verbose

Just run some experiments...

> cat 705063.pl
my $verbose = (@ARGV && $ARGV[0] eq '-v' && shift);
if (defined $verbose) {
    print "verbose:$verbose:\n";
}
else {
    print "verbose not defined\n";
}
> 
> ./705063.pl
verbose:0:
> ./705063.pl -v
verbose:-v:
> ./705063.pl -v -b -n
verbose:-v:
> ./705063.pl -b
verbose::
[download]

If no arguments are passed to the script, verbose will be 0.

If any arguments are passed to the script, and the 1st is NOT '-v', then verbose seems to be a zero-length string, but not 'undef'. Perhaps a more knowledgable Monk can explain why.

If any arguments are passed to the script, and the 1st is '-v', then verbose is '-v'.

If this behavior is not what you want and you have control over the code, I suggest re-writing it to achieve what you want in a way that you understand.