you need a combination of two of the replies:

foreach my $element ( keys %dateparts ) {
    die "No datepart!: $!" unless defined $dateparts{$element};
}
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the 'exit' is redundant. if the code dies, the program is exiting.

i also don't think your assignment is going to work correctly. if you're passing in date values, you really should just use something like:

my %dateparts = @_;
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assuming that your input args are in the form of:

sub MySQLIN( YEAR => '1999', DAY => '10', MONTH => '3');
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the join will work even if the values to join are undefined.