When I set a break point inside a loop and I want to track the value of one or more variables at each iteration, I find it saves a lot of time/tedium to use "< expression" -- like so:

 DB<1> b 123
 DB<2> < print "## $var1 ## $var2 ## $whatever ##\n"
 DB<3> c
123:      some_line_of_code();
## var1_value ## thisisvar2 ## and so on ##
 DB<4>
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Naturally, you could put "print Dumper($whatever)" as well, or any other operation that might speed up the debugging inside a loop.