A return() statement exits the subroutine. That means you loop won't be completed, since incStart() will stop and return at the first value that matches ( $x % $_[2] ) == 0.

You'll want to create a list of matching variables to return. You could do it a bit like this: (code clarified a bit)

use strict;
use warnings;

sub incStart {
    my ($start,$end,$mod) = @_; 
    my @result;
    for ( my $x = $start ; $x < $end ; $x++ ) {
        if ( ( $x % $mod ) == 0 ) {
           push @result,$x;  # append $x to result list
        }
    }
    return @result;  # <- return AFTER completing the loop
}

print "test ";
foreach my $result (incStart(0,10,3)) {
   print "$result\n";
}
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Please also be very careful about where you define variables and use strict and warnings.