'OP\'s' => sub {
     my $lineno = 100;
     open( FILE, "$f" ) or die "Can't find $f\n";
     my @lines = <FILE>;
     my $num = @lines;
     for ( ; $lineno > 0 ; $lineno-- ) {
         my @tail = @lines[ $num - $lineno ];
     }
     close(FILE);
},
[download]

This can't be correct. You never create an array with 100 lines. You do create 100 arrays with one line each, using a single element slice ('use warnings' complains).

'`tail -100`' => sub {
    my @lines = split( /\n/, `tail -100 $f` );
},
[download]

Why the explicite splitting? Why not just

    my @lines = `tail -100 $f`;
[download]

Not that it makes a huge difference speedwise.