I think it would be best to split the record on the escape character, convert each part to UTF8 separately and then merge the results back. Something like

my $record = "a" . $record; # prepend the ID of the default charset
my @parts = split /\x1B/, $record;
foreach my $part (@parts) {
 my $charset = substr( $part,0,1,'');
 $charset = $ESC{$charset}; # to get the name from the id
 $part = decode( $charset, $part);
}
$record = join '', @parts;
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