# assuming 5.8.1 or later:

perl -C9 -pe 's/([^[:ascii:]])/sprintf("\\x{%.4x}",ord $1)/eg' sometext.utf8

# that is, leave ascii data as-is, convert wide characters to "\x{HHHH}"

##</code><code>##

perl -pe 'BEGIN{binmode STDIN,":encoding(iso-8859-5)"} s/([^[:ascii:]])/sprintf("\\x{%.4x}",ord $1)/eg' < iso-cyrillic.txt