I think the following is much easier to read than using map:

sub list {
  my $words=shift;
  my @words=split /\s/, $words;
  my @rv;
  for my $s (0..$#words) {
    for my $e ($s..$#words) {
      push(@rv, join(' ', @words[$s..$e]));
    }
  }
  return @rv;
}
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And you can keep the optimization yours had:

sub list {
  my $words=shift;
  my @words=split /\s/, $words;
  my @rv = @words;
  for my $s (0..$#words) {
    for my $e ($s+1..$#words) {
      push(@rv, join(' ', @words[$s..$e]));
    }
  }
  return @rv;
}
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