There are a number of problems with this regex. If you are trying to parse some HTML or similar markup, this is REALLY NOT THE WAY TO DO IT, even if it were doing what you want! But I strongly suspect is doesn't do what you think it does. Consider:

use strict;

my @testStrs = ("all this<form>", "meh!<form>");

for my $str (@testStrs) {
    my $contents = $str;

    $contents =~ s/^[^<form]*(?=<form)//smg;

    print qq(After: "$contents" Before: "$str"\n);
}
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Prints:

After: "<form>" Before: "all this<form>"
After: "meh!<form>" Before: "meh!<form>"
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[^<form] matches any one character that is not '<', 'f', 'o', 'r' or 'm'. Guessing at what you may actually want to do, onsider instead:

use strict;
use warnings;
use HTML::TreeBuilder;

my $htmlFrag = ("<div><form>All good men</form></div>");
my $root = HTML::TreeBuilder->new_from_content($htmlFrag)->elementify(
+);
my $form = $root->look_down("_tag", "form");

print $form->as_text();
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which prints

All good men
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See HTML::TreeBuilder and HTML::Element.