That error message means that you have a string (specifically " '{print $1,$2 }' |") where a string shouldn't be. While you intended to write only one string for the piped open, you have two strings, thanks to the field separator passed to awk. This is a good use of an alternate quoting construct:

open my $IN, q!ps -aux | awk -F" " '{print $1,$2 }' |!
    or die "Can't open pipe";
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I switched to the non-interpolating form because you probably don't want to interpolate the regexp match variables $1 and $2 into your string there.

Of course, you could skip the pipe to awk and process the result with Perl (split) will come in handy....