#!/usr/bin/perl -wl

    use strict;

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

    $str    =~ s|({[^}]+})|(my $tmp = $1) =~ s/ /_/g; $tmp|eg;
    print $str;

    __END__
    Prints:

    {fred and barney} {bam bam} {pebbles} {} {   }
    {fred_and_barney} {bam_bam} {pebbles} {} {___}