You can just use the named captures $1 and $2 to store the values (also note the warning in perlre about (?{ code }) being highly experimental).. also, no need for string references here. 
use strict;
use warnings;

while (<DATA>) {
    next unless /\S/;
    my ( $sunrise, $sunset ) = get_time($_);
    next unless $sunrise && $sunset;
    printf "Sunrise is: %s Sunset is: %s\n", $sunrise, $sunset;

}

sub get_time {
  my $string = shift;
  return unless $string =~ /sunrise:\s+(\d+:\d+)\s+sunset:\s+(\d+:\d+)
+/;
  return ( $1, $2 );
}

__DATA__
Aberdeen, Scotland 57 9 N 2 9 W sunrise: 03:12 sunset: 21:08
Adelaide, Australia 34 55 S 138 36 E sunrise: 06:52 sunset: 16:41

[download]
But where the undef was coming from was a blank line in <DATA> ... so the match failed, so $sunrise/$sunset where undef. Just a quick check for a valid string/success on the match resolves it.

In reply to Re: Assigning a varable inside of (?{}) by davidrw
in thread Assigning a varable inside of (?{}) by monsterzero

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