if ($_ = (m/^[\$]\w/)) {
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Solution pointed out below. This is an assignment (using =) clobbering $_ with an empty array the true/false result. An assignment of an undefined thing evaluates to false every time. Similar to if ($_ = ()) or if ($_ = undef).

Update - because I was curious:

perl -MO=Deparse,-P -e 'if($_ = (m/foo/)) { print 'hi' }'
if ($_ = /foo/) {
    print hi $_;
}
-e syntax OK
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A little more insight.

update 2- corrected ty