You say that you "need /s so . to match newlines as well", and I assume this means that if your pattern list includes something like foo.*?bar, you would want that to match either of the following examples (where I'll put parens around the intended match):

example 1: blah (foobar) blah

example 2: blah (foo
all sorts of content on
lots of lines
of data 
bar) blah
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In that case, you need to read the data file in slurp mode:

my $regex_string = join '|', @list_patterns;

open( FILE, "<", $arg1 ) or die "$arg1: $!";
$_ = do { local $/; <FILE> };  # temporarily set $/ = undef to slurp f
+ile
close FILE;

if ( /($regex_string)/is ) {
    # got a match...
}
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Regarding the placement of parens and regex qualifiers, you could do that in other ways, like:

my $list_regex = qr/($regex_string)/is;
...
if ( /$list_regex/ ) {
...
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No difference, really, but when there's a choice, I like having the parens visible in the code near the place where I use $1, $2, etc.