B::Deparse tells perl to spit back what it parsed your code as, optionally adding parens which you may have omitted to make precedence clearer.

As for =~, it does say match against what's on the left hand side; the problem in this case is that what you had on the right hand side wasn't a simple m// so it was treated as an expression yielding a regexp object as if you'd used qr//. In other words, what you did was more:

$foo = /^#+/;    # match against $_. a false '' since you never set $_
$bar = not $foo; # take the logical opposite, so always "1"
$baz = qr/$bar/; # make a regep object from the contents matching "1"
$one_line =~ m/$baz/; # returns true if $one_line has a 1
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