my $SAVE_PLACE;
GetOptions(
    's|save_to:s' => \$SAVE_PLACE,
);

# here the value of $SAVE_PLACE will be:
# undef if no -s --save_to
# 0 if -s --save_to but no optional argument
# <str> if -s=<str>

while ( defined $SAVE_PLACE and not $SAVE_PLACE ) {
    $SAVE_PLACE = prompt("Save to where: ");
}

sub prompt { print @_; chomp(my $ans = <>); $ans }