An s flag tells the regex to ignore line boundaries, which it won't do normally.

I wouldn't put it that way. /s makes newlines a valid match for ".". Moreover, perl normally does ignore internal line boundaries unless one tells it not to with /m. E.g.

my $s = "a\nb\nc\n";
my @g   = map "[$_]", $s =~ /(.)$/g;
my @gs  = map "[$_]", $s =~ /(.)$/gs;
my @gm  = map "[$_]", $s =~ /(.)$/gm;
my @gsm = map "[$_]", $s =~ /(.)$/gsm;
print "g:   @g\n";
print "gs:  @gs\n";
print "gm:  @gm\n";
print "gsm: @gsm\n";
__END__
g:   [c]
gs:  [c] [
]
gm:  [a] [b] [c]
gsm: [a] [b] [c] [
]
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Note in particular the case in which both /s and /m are used.