use strict;
 
my %hash = (
   one => 'blah',
   two => 'echo',
   thr => 'aaaak',
);
 
foreach my $key (keys %hash) {
  $hash{"~foo".$key} = $hash{$key};
  delete $hash{$key};
}
 
use Data::Dumper;
print Dumper(\%hash);