I guess you haven't used warnings. If you do, then a helpful warning is displayed. viz:

$ cat posix.pl 
#!/usr/bin/env perl 
use strict;
use warnings;

my $str = "ab\x{cc}";
$str =~ s/(\P{PosixPrint})/sprintf("U+%04X", $1)/eg;
print "$str\n";
$ ./posix.pl 
Argument "Ì" isn't numeric in sprintf at ./posix.pl line 6.
abU+0000
$
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So the warning indicates that a numeric argument is expected but that you are providing a non-numeric value. We can fix this with ord:

#!/usr/bin/env perl 
use strict;
use warnings;

my $str = "ab\x{cc}";
$str =~ s/(\P{PosixPrint})/sprintf("U+%04X", ord $1)/eg;
print "$str\n";
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The moral: Always use strict and warnings.