File::Compare

or

compare( 'invoices.txt', 'invoices.bak' );

sub compare
{
    local *FH_A;
    local *FH_B;
    open FH_A, "< $_[0]\0" or die "Can't open $_[0] for reading: $!";
    open FH_B, "< $_[1]\0" or die "Can't open $_[1] for reading: $!";
    binmode FH_A or die "Can't binmode $_[0]: $!";
    binmode FH_B or die "Can't binmode $_[1]: $!";

    local $/ = \ 4096;
    1 while <FH_A> eq <FH_B>;
    return eof *FH_A and eof *FH_B;
}
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