by using strict and warnings, or by putting some print 
statements

#! /usr/bin/perl -d

we can also use -c and -d switches to debug our perl scripts
ex:
perl -c sample.pl (for compilation errors)
perl -d sample.pl (for debugging the code line by line)

For debugging perl scripting u can use 
1. diagnostic modules - which  can help us isolate and
understand problems with our code.

EX- use warning, use strict, use diagnostics 

2.perl command line switches - u can create test programm
using the perl command line.

Ex-i) #!user/bin/perl -w
or on the command line like this

>perl -w program.plx

ii) -e = it tells the perl not to load and run a program
file but to run the text following -e as a program.

 > perl -e "print \"hello World \n";" 

iii)-c = stops perl from running your program instead ,al it
does is check that code can be complied as valid perl.
its a good way to quickly check that a program has no
glaring syntax errors.
> perl -ce 'print "hello,world\n";'
and so on it will available Google u can search ........
3. debugging techniques and perl de-debugger - how to remove
the problems that we've found.

 Using debugger
> perl -d filename.plx