use Time::HiRes qw(gettimeofday tv_interval);

my $bigfile = shift or die "Big file?";
sub openandreadbigfile {
    open my $fh, "<:raw", $bigfile or die $!;
    local $/;
    my $xxx = <$fh>;
    undef;
}

while () {
    my $t0 = [gettimeofday];
    openandreadbigfile();
    my $elapsed = tv_interval($t0);
    warn $elapsed, "\n";
}