$ perl -MEncode=_utf8_on -e'$s = "\xF1\xF4"; _utf8_on($s); "" =~ /$s/'
Malformed UTF-8 character (unexpected non-continuation byte 0xf4, immediately after start byte 0xf1) in regexp compilation at -e line 1.
Malformed UTF-8 character (1 byte, need 4, after start byte 0xf4) in regexp compilation at -e line 1.
Malformed UTF-8 character (unexpected non-continuation byte 0xf4, immediately after start byte 0xf1) in regexp compilation at -e line 1.
Malformed UTF-8 character (1 byte, need 4, after start byte 0xf4) in regexp compilation at -e line 1.

$ perl -MEncode=_utf8_on -e'$s = "\xF1\xF4"; "" =~ /$s/'

$