For better performances you'd probably prefer to use a translation instead of a substitution.

perl -MBenchmark -e 'timethese(1000000,{
   "s"  => "$_=qq(a a a a a a ); s/ /_/g",
   "tr" => "$_=qq(a a a a a a ); tr/ /_/",
})'

Benchmark: timing 1000000 iterations of s, tr...
 s: 15 wallclock secs (13.81 usr +  0.00 sys = 13.81 CPU) 
      @ 72411.30/s (n=1000000)
 tr:  5 wallclock secs ( 4.38 usr +  0.00 sys =  4.38 CPU) 
      @ 228519.20/s (n=1000000)
[download]

<kbd>--
my $OeufMayo = new PerlMonger::Paris({http => 'paris.mongueurs.net'});</kbd>