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my $limit = 100;
my $file  = 'big.file';

...
    return @{$current == \@alpha ? \@beta : \@alpha}[-($limit - scalar
+ @$current)..-1],
        @$current;
}

% perl tailbm -30 10 100.lines 

...
   grinder:  74 wallclock secs (28.38 usr +  2.77 sys = 31.15 CPU) @ 5
+66.52/s (n=17647)
     lastn:  69 wallclock secs (19.78 usr + 11.34 sys = 31.12 CPU) @ 2
+2.88/s (n=712)
lastn_getc:  77 wallclock secs (18.88 usr + 13.17 sys = 32.05 CPU) @ 1
+9.78/s (n=634)

% perl tailbm -30 100 721994.lines 
Benchmark: running f_rb_obj, f_rb_obj_u, f_rb_tie, f_rb_tie_u, file_ta
+il, grinder, each for at least 30 CPU seconds...
...
            (warning: too few iterations for a reliable count)
     lastn:  40 wallclock secs (20.15 usr + 10.80 sys = 30.95 CPU) @ 1
+9.48/s (n=603)
lastn_getc:  42 wallclock secs (18.05 usr + 13.81 sys = 31.86 CPU) @ 1
+7.70/s (n=564)

#! /usr/bin/perl -w

...
    'lastn_getc' => \&lastn_getc, 
});
__END__