$ perl -wE 'say for qr/(a|b|x)/i, qr{ /(a|b|x)/}i'
(?^ui:(a|b|x))
(?^ui: /(a|b|x)/)
[download]

Moreover, $_ = /.../ not only matches against $_, but also changes its content (it sets it to the result of the match).

Also note that my $_ is invalid since 5.24.