A few minor points.

You can use splice instead of your multiple shifts. Replace

shift @dates;
shift @dates;
shift @dates;
shift @dates;
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with

splice @dates, 0, 4;
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If you are reading a file that is supplied to your script on the command line you can read it by doing <> rather than having to explicitly say <STDIN>

When reading from the file you might be better off reading a line at a time, especially if the file is large.

...
my @firstLine = <>;
my @dates = split m{\|}, $firstLine;
...
while ( my $line = <> )
{
...
}
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I hope this is of use,

Cheers,

JohnGG