printf ("%lX ", $buffer);

You probably want ord:

printf ("0x%lX ", ord $buffer);
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because what you have in $buffer is a single byte string, not a number.

A more direct way might be to use unpack "H*", ...

$ perl -E'say unpack "H*", "foobar"'
666f6f626172
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or maybe unpack "(H2)*", ..., as in

$ perl -E'say "0x$_" for unpack "(H2)*", "foobar"'
0x66
0x6f
0x6f
0x62
0x61
0x72
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in which case your buffer could consist of more than one byte.