Don't declare the $x — it's just a one-liner. Perl wraps a while(<>) { ... } loop around the code you put there (really!), so it's a new $x for every line in the file.

See for yourself:

perl -n -MO=Deparse -e 'my $x' foo
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produces:

LINE: while (defined($_ = <ARGV>)) {
    my $x;
}
-e syntax OK
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