You can use a negated character class to match up until the included chars in the class are found.

#!/usr/bin/env perl
use strict;
use warnings;
use feature 'say';

my @test_cases = ( 'KP_what_I_want(but_not_this',
                   'KP_what_I_want nor_this', 
                   'KP_ nor_any_of_this',
                   'and this will not match',
                 ); 
             
for my $line (@test_cases) {

    my ($exp) = $line =~ m/ KP_   # Required.
                         ( [^ (]* ) # Capture until space 
                                    # or left paren.
                         /xi;

    if (defined $exp) {say "<$exp>"}
        else {say 'NO MATCH'};
}
__END__
<what_I_want>
<what_I_want>
<>
NO MATCH
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