Update: I should have thought a bit more before pressing "submit".. "list" at the beginning of the line is part of the problem statement. I would be thinking of removing that, then doing the match global on what is left as shown below.  /^list\s+(\w+)/g won't work, only gets the foo, not also the bar. I think that there is some way to get match global to back up and keep capturing (\w+)'s, in a single regex, but at the moment, I don't know how to do that. Sorry.

Update 2: now another solution occurred to me, that appears to work:
(updated code to add: f("foo,bar,bar2,bar3"); - no regex change, just another data example)

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
sub f {
    my $in = shift;    
    my (@tmp) = "list $in" =~ /(?:^list\s+)?(\w+)/g;
    print(Dumper(\@tmp));
}
f("foo");
f("foo,bar");
f("foo,bar,bar2,bar3");

__END__
$VAR1 = [
          'foo'
        ];
$VAR1 = [
          'foo',
          'bar'
        ];
$VAR1 = [
          'foo',
          'bar',
          'bar2',
          'bar3'
        ];
[download]

One way is to use match global:

#!/usr/bin/perl
use warnings;
use strict;
use Data::Dumper;
sub f {
    my $in = shift;    
    my (@tmp) = $in =~ /(\w+)/g;
    print(Dumper(\@tmp));
}
f("foo");
f("foo,bar");

__END__
$VAR1 = [
          'foo'
        ];
$VAR1 = [
          'foo',
          'bar'
        ];
[download]