The following should probably do what you want:

minimum($dataA->xchg(0,1));
maximum($dataB->xchg(0,1));
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minimum() and maximum() take the minimum and maximum values over the first dimension. As you need the these values from the second dimension, you can use xchg to swap dimensions.

See also: http://pdl.sourceforge.net/PDLdocs/Ufunc.html#minimum

Your original code should also work, if you use double quotes instead of single ones:

for(my $k=0; $k<$maxColumns; $k++) {
    print "Min: " . min($dataA->slice("$k,:")) . "\n\n";
    print "Max: " . max($dataB->slice("$k,:")) . "\n\n";    
}
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-L.