You could explicitly use the concatenation operator . and do $distlist .= " $k\@myorg.mydomain".

You could build an array and join the items after the fact.

my @addrs;
for my $x ( @emaillist ) {
  push @addrs, "$k\@myorg.mydomain";
}
my $distlist = join( " ", @addrs );
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You could use map and join and do it all in one swell foop.

my $distlist = join( " ", map { "$_\@myorg.mydomain" } split( /,/, $AR
+GV[1] ) );
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Depends on one's definition of better.