> perl -l
$,=$/;
print 'right:', qr(a\@b), qr(a\Q@\Eb), qr(\Qa@\Eb), qr(\Qa\E\@\Qb\E);
print 'wrong:', qr(\Qa@b\E), qr(\Qa\@b\E);
__END__
right:
(?-xism:a\@b)
(?-xism:a\@b)
(?-xism:a\@b)
(?-xism:a\@b)
wrong:
(?-xism:a)
(?-xism:a\\\@b)

##</code><code>##

> perl -l
$_='root: admin@somewhere.here';
print '(',join(")(",/^(root:)(\s*)(?!admin\@somewhere\.here)/),')';
print qq[Postmatch contained "$'"];
__END__
(root:)()
Postmatch contained " admin@somewhere.here"

##</code><code>##

Hardcoded:
/^root:(?!\s*admin\@somewhere\.here)/

Variable:
my $admin_email = 'admin@somewhere.here';
/^root:(?!\s*\Q$admin_email\E)/