my %prefixList=(
   3 => {DEF=>1, SRC=>1},
   6 => {ABCD12=>1},
  10 => {DEEEE11223},
);

#....
# inside the loop
foreach my $length(%prefixList) {
    if($prefixList{$length}->{substr($fields[7],0,$length)}){
        ### code here
        last;
    }
}