You can tell if the case-insensitive modifier was used pretty easily:

sub is_ci {
    my ($pat) = @_;
    my ($pos_mods) = $pat =~ /^\(\?(\w*)(?:-\w*)?:.*\)/s
        or return 0;
    return $pos_mods =~ /i/;
}
[download]

Or even the following in Perl ≥ 5.10:

use re qw( regexp_pattern );
sub is_ci { (regexp_pattern(shift))[1] =~ /i/ }
[download]

But you can't tell if a pattern is case-sensitive or not.

The following will all come up as case-sensitive even though they're not:

qr/(?i:hello)/
qr/(?i)hello/
qr/[hH][eE][lL][lL][oO]/
qr/1234/
[download]

The following will all come up as case-insensitive even though they're not:

qr/(?-i)hello/i
qr/(?-i:hello)/i
qr/he(?-i:l)lo/i
[download]

Update Forgot to escape ? in my pattern. Fixed.