my $str2 =  '{fred and barney} {bam bam} {pebbles} {} {   }';
    print $str2;

    my $var  = "xxxxx";

    $str2    =~ s|({[^}]+})|{$var}|g;
    print $str2;

    __END__
    Prints:

    {fred and barney} {bam bam} {pebbles} {} {   }
    {xxxxx} {xxxxx} {xxxxx} {} {xxxxx}