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open (my $file, '<', 'data.bin');
binmode $file;
...
$binary_data =~ /(.)(.){(?{unpack 'H*', $1})}/; # ?
# or may be
/(.)(?{unpack 'H*', $1})(.){$^R}/; # ???

/(.)((??{'.' x $1}))/;

$_ = "3aaaa\nbbbb\nccccc\nddddd";
/(\d)((??{'.*?\n' x $1}))/;
print $2; # aaaa\nbbbb\nccccc\n
# how to assign a value aaaa\n of $3, bbbb\n -> $4 etc ?