Here's some example code that might help you understand.

#!/usr/bin/perl

$x ="somestringyoulike";   

$y = pack("p", $x);  # Null terminated
$z = pack("P",$x);   # Fixed length

print '$x length  : ', length($x), "\n";
print '$y length  : ', length($y), " (it's a pointer.)\n";
print '$z length  : ', length($y), " (it's a pointer.)\n";

print 'unpack $y  : ', unpack("p",$y), "\n";

$x = "a new string";
print 'unpack $y  : ', unpack("p",$y), "\n";    # $y still points to $
+x


print '$z  1 byte : ', unpack("P",$z), "\n";     # The first byte of $
+z
print '$z  4 bytes: ', unpack("P4",$z), "\n";    # The first 4 bytes.
print '$z 17 bytes: ', unpack("P17",$z), "\n";   # The whole 17 bytes.

# Remember that $y is null terminated... so there must be
# a null in the string when we unpack 17 bytes of $z 
# because they point to the same thing. This finds that null...
$s = unpack("P17", $z);
$s =~ /(.)\0(.)/;
print '\0  between: ', "$1 and $2\n";
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-sauoq
"My two cents aren't worth a dime.";