Just want to point out that (nested) while (<WHATEVER>)'s clobber $_ because of no localization/lexicalization:

while (<DATA>)
{
    print; # $_ is "line 0001\n"

    open(my $FH, "<", "file") or die($!);
    while (<$FH>) { ... }

    # $_ was clobbered by: while (<$FH>)
}

__DATA__
line 0001
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For a better explanation, read Sins of Perl Revisited.