substr can be used to extract portions of other strings. It returns a string, though. If you want to treat each character of that string as a byte, you can then use unpack 'C*'.

Square brackets can be used to create anonymous arrays. The contents of the brackets are assigned to the array. The operator returns a reference to the anonymous array.

$array[2][3] gets the 4th element of the array referenced by the 2nd element of @array. perllol

All together:

my @recs;
while (read($fh, my $rec, 158)) {
    push @recs, [ unpack 'C*', substr($rec, 22, 13) ];
}

print $recs[2][3];
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Since you're using unpack anyway, you can just skip over the unwanted bytes instead of using substr.

    push @recs, [ unpack 'x22 C13', $rec ];
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Update: Ooops, was missing the "*" in the unpack format.