my $str= "1_abc/2_def/bla_30_31_blah";
my $count= 0;
while(  $str =~ /\d+_/g  ) {
    last   if  2 < ++$count;
    $str =~ s///;
}
print "\$str=($str)\n";
# $str=(abc/def/bla_30_31_blah)