$s ='25 {fred and barney} text 2.36 12.0 {bam bam} text {pebbles}';
$s =~ s[(\{.*?\})]{ local $_=$1; tr[ ][_]; $_}ge;
print $s;
25 {fred_and_barney} text 2.36 12.0 {bam_bam} text {pebbles}