You could separate the parts of the unpack format, making sure to manually consume each part as you go along. It's more work than should be necessary of course - unpack should tell us where it failed, but alas, it doesn't. So you could repeatedly call something like

my @list = eval { unpack "x$skip $format", $data };
{ die "$@" || last }
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making sure that $skip contains the correct value between calls.

Makeshifts last the longest.