From Extended Patterns:

(?^alupimsx)
One or more embedded pattern-match modifiers, to be turned on (or turned off, if preceded by -) for the remainder of the pattern or the remainder of the enclosing pattern group (if any).

and

Starting in Perl 5.14, a "^" (caret or circumflex accent) immediately after the "?" is a shorthand equivalent to d-imsx.

So, (?^u:abc) is shorthand for (?du-imsx:abc), which means (see Modifiers), match the string abc with:

• /d    “the old, problematic, pre-5.14 Default character set behavior”;
• /u    using the Unicode character set;
• -i    with case-sensitive pattern matching;
• -m    the string not treated as multiple lines ("^" and "$" match at the start or end, respectively, of the string);
• -s    the "." pattern not matching the newline character; and
• -x    whitespace and comments treated as patterns to be matched.

Hope that helps,