unpack or sprintf to get the hex digits. The substitution operator to match the hex digits, prepending each pair with the literal '\x'.

my $string = "39860000N000";

my $hexlen = sprintf( '%4.4x', length $string ) =~ s/ ( \p{Hex}{2} ) /
+\\x$1/grx;

print "Length of ($string) in hex: ($hexlen)\n";
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In older versions of Perl that didn't have the /r flag you would probably want to break that into a couple of statements (and may still wish to do so for clarity's sake):

my $string = '39860000N000';
my $hexlen = sprintf '%4.4x', length $string;
$hexlen =~ s/  ( \p{Hex}{2} ) /\\x$1/gx;
print "Length of ($string) in hex: ($hexlen)\n";
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