You're prossibly interested in watch expressions set with something along the lines of w $var which will show the old and new value whenever the watched expression changes values (and that can be an expression, not just a single variable; whenever the value of that expression is different the watch triggers). There's not much extra detail in the docs but if you search perldebug you can find this and play with a short test script:

    w expr      Add a global watch-expression. Whenever a watched glob
+al
                changes the debugger will stop and display the old and
+ new
                values.
[download]

Alternately you might be interested in a conditional breakpoint which will only drop into the debugger if a supplied expression evaluates to true (unlike the typical unconditional breakpoint you get with the normal b LINE that always stops at line number LINE).