ranged conditional with variable

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

I'm attempting to do the following, be receive the following error: "Use of uninitialized value in pattern match (m//)"

foreach my $line (<FILE>) {
    if ($line =~ /PatternBegin/ .. /PatternEnd/) {
        $str .= $line;
    }
}
[download]

If i remove the variable declaration and work from $_ with the following code everything works fine.

foreach (<FILE>) {
    if (/PatternBegin/ .. /PatternEnd/) {
        $str .= $_;
    }
}
[download]

How do I make it work with the first example of code?