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

print "before - $str\n";

$str =~ s/{ ([^}]+) }/{$rep}/xg;

print "after  - $str\n";

__output__

before - 25 {fred and barney} text 2.36 12.0 {bam bam} text {pebbles}
after  - 25 {between brackets} text 2.36 12.0 {between brackets} text {between brackets}