It's simple:

use Encode qw(decode);

my $input_string = 'some octets';
my $input_encoding = 'cp1250';  # or 'iso-8859-1', or 'shiftjis' or wh
+atever

my $utf8 = decode($input_encoding,$input_string);
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Note that there is no reliable way of determining $input_encoding automatically.