my $str =
  "25 {fred and barney} text 2.36 12.0 {bam bam} text {pebbles}";

print "before - $str\n";

substr($str, pos($str) - length($1), length $1) =~ s/ /_/g
  while $str =~ /\G .*? { ( [^}]+ ) } /xg;

print "after  - $str\n";

__output__

before - 25 {fred and barney} text 2.36 12.0 {bam bam} text {pebbles}
after  - 25 {fred_and_barney} text 2.36 12.0 {bam_bam} text {pebbles}