$s = "foo\nfoot\nroot";
$s =~ /^foo/g;           # matches only the first foo
$s =~ /^foo/gm;          # matches both foo
$s =~ /f.*t/g;           # matches only foot
$s =~ /f.*t/gs;          # matches foo\nfoot\nroot
$s =~ /f.*?t/gs;         # matches foo\nfoot
$s =~ /^foot.*root$/g;   # doesn't match
$s =~ /^foot.*root$/gm;  # doesn't match
$s =~ /^foot.*root$/gs;  # doesn't match
$s =~ /^foot.*root$/gms; # matches foot\nroot