Not certain that its exactly a great Benchmark, but it seems to clearly indicates that using the x op is substantially faster.

#! perl -sw
use strict;
use Benchmark qw(cmpthese);

sub mapgen    { return map $_[0], (1..$_[1]); }
sub xgen    { return ($_[0]) x $_[1]; }

cmpthese( 1000, {
    mapgen    => 'my @ones = mapgen 1, 1000;',
    xgen    => 'my @ones = xgen 1, 1000;',
});

__END__
Benchmark: timing 1000 iterations of mapgen, xgen...
    mapgen:  9 wallclock secs ( 8.18 usr +  0.00 sys =  8.18 CPU) @ 12
+2.23/s (n=1000)
      xgen:  5 wallclock secs ( 5.15 usr +  0.00 sys =  5.15 CPU) @ 19
+4.29/s (n=1000)
        Rate mapgen   xgen
mapgen 122/s     --   -37%
xgen   194/s    59%     --

C:\test>
[download]