Here is one way using a negated character class:

    #!/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} {} {___}
[download]

To substitute a variable you could modify the above as follows:

    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}
[download]

If you have to deal with nested braces you should have a look at the Text::Balanced module.

--
John.