bar() if my $foo;
is different from
my $foo if bar(); # don't do this!

The first case is equivalent to

if (my $foo) {
    # $foo is lexically scoped to this block
    bar();
}
[download]

for my $foo (@foo) {
    # $foo is lexically scoped to this block
    baz();
}
[download]

So if ((my $start =~ /start/) .. (my $end =~ /end/)) is perfectly happy.