# Assuming you already have read/set the values for $output_dir and $map_file...
# and a list of project.txt files is given in @ARGV
use File::Spec::Functions qw(rel2abs);
use File::Basename qw(dirname);
while(<>) {
    if(/^REVISION_LOCATION:(.*)/) {
        # $ARGV contains the name of the project.txt file you're reading
        system($otherscript, "--root=".rel2abs($1, dirname($ARGV)), "--outdir=$out_dir", "--map=$map_file");
    }
}