use Encode;

...
# assuming $data is a scalar containing a utf8 string:
$data = encode( "cp1250", $data );

# now it contains a cp1250 string
# (utf8 flag is off, and characters are single-byte)

##</code><code>##


open( OUT, ">:encoding(cp1250)", "output.txt" ) or die "output.txt: $!\n";

# assuming $data contains a utf8 string:
print OUT $data;

# the string gets converted to cp1250 before being written to the file
# (no need to "use Encode" in this case)