Congratulations, you have stumbled onto autovivification. :-) To expand on swkronenfeld’s point, this always happens when you dereference an undefined value “en passant”:

undef $_;
$_->[0];
print $_;
__END__
ARRAY(0x812f180)
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This is so you can build deep structures without a lot of checks:

my %bill;

while( <$records_fh> ) {
    my ( $time, $worker, $project ) = split /\t/, $_;
    push @{ $bill{ $project }{ $worker } }, $time;
}
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If Perl didn’t autovivify, you would have to write this like so:

my %bill;

while( <$records_fh> ) {
    my ( $time, $worker, $project ) = split /\t/, $_;

    $bill{ $project } = {} if not exists $bill{ $project };

    $bill{ $project }{ $worker } = []
        if not exists $bill{ $project }{ $worker };

    push @{ $bill{ $project }{ $worker } }, $time;
}
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Most of the time this implicit creation of anonymous data structures on access is very convenient. However, you have to be mindful of it when you are writing condition checks.

Makeshifts last the longest.