You might not be able to do this for all regexes, and like Abigail-II says, its a costly operation, but in your particular case, you could do something like this:

my $str = "abc def ghi abc 123 junk";
my $short;          
while ($str =~ /abc.*?123/g) {
  $short = $& if ! defined $short or length($&) < length($short);
  pos($str) = $-[0]+1;  
}
print "$short\n";
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