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my $dir = "/Path/to/a/data/directory";
my %hash;
...

    return { $key => \@values };
}

my $dir = "/Path/to/a/data/directory";
my %hash;
...
    %{$hash{$file}} = (%{$hash{$file}}, %{hashify_line( $line )});
}
close $grep_fh or die "Error closing grep pipe: $!\n";

my $cmd = "find $dir -type f -print | xargs grep '^' /dev/null";
open my $grep_fh, '-|', $cmd
    or die "Can't grep: $!\n";