local will do this for you automatically, if you scope it correctly:

my $cat2;
{
    open(FH, 'data.txt') || die "Some error: $!";
    local $/;
    $cat2 = <FH>;
    close FH;
}
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You have to make sure $cat2 is defined outside of the block, though, otherwise it'll go out of scope when you want to use it.