First, you should be running with warnings and strict!

#!/usr/bin/perl -w
use strict;
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This provides HUGE clues as to what might be wrong!

I think you'll find that the /\s+/ hint by graff is needed and also consider:

$min = shift(@positions);
$max = pop(@positions);
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What happens if min and max are the same? i.e. just one position?

$max = (@positions)[-1];
$min = (@positions)[0];
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will handle that situation.

Update: a small update, also keep in mind that list slice allows multiple values to the left hand side, my ($min,$max) = (@positions)[0,-1]; would work also. The -1 index means the last one in the array, -2 would be second to last etc. But FAR AND AWAY, the best thing you can do to improve your code is religious use of warnings and strict!