Thank you Paul for your very prompt reply.

And the example you posted evaluates for me also. I think that clearly
exonerates your module from any blame. My code is another matter.
For curiosity I'll post a snippet of my code. As the whole script is
around 47kB this is just a snip, and it's entirely possible the error
is elsewhere.

<code>
#!/usr/bin/perl -w

use Math::Business::DMI;

#declarations
my $dmi = Math::Business::DMI->recommended;
my (
        @dmiData,
        @dmiDataSlice,
        $adx,
        $pdi,
        $mdi,
);

#part of main loop
@dmiDataSlice = (
        priceToTick($data->{$selectionId}->{'B1'}->{'price'}),
        priceToTick($data->{$selectionId}->{'L1'}->{'price'}),
        priceToTick($data->{$selectionId}->{'runner'}-

 );

        push (@dmiData, [@dmiDataSlice]);
        if ( scalar (@dmiData) >5) {
                shift @dmiData;
        }

        print "@dmiDataSlice\n"; # debug to check the input data

        $dmi->insert( @dmiData );
        ($pdi, $mdi, $adx) = $dmi->query;

#loop
</code>
The whole script features a loop which reads prices twice a second and
places trades depending on position. I intended to add in the DMI
module to indicate a strong position, either buy or sell.
The priceToTick function returns an integer

This snippet is supposed to push new data onto the @dmiData array and
shift old data off it.
I have managed to get it to run without dying by changing the '5' to
'4' in the following line:

        if ( scalar (@dmiData) >5) {

but as to why I'm currently none the wiser! I suspect it's probably
inexperience with Perl in general, and referencing/dereferencing
arrays in particular.