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open (IN, "file1") or die("can't blah blah");
while (<IN>) {
...
    my $slurp = do { local $/ = undef; open IN, "file2"; <IN>};
    return $slurp;
}

open (my $in, "file1") or die("can't blah blah");
while (<$in>) {
...
    my $slurp = do { local $/ = undef; open my $in, "file2"; <$in>};
    return $slurp;
}