from perldoc -f exec:

exec LIST
exec PROGRAM LIST
   The "exec" function executes a system command and never returns-- u
+se "system" instead of "exec" if you want it to return. [...]
Since it's a common mistake to use exec instead of system, Perl warns 
+you if there is a following statement which isn't die, warn, or exit 
+(if -w  is set - but you always do that).
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You should always turn on -w flag or "use warnings" (moreover if you are trying to debug your code!)

perl -e 'use warnings;exec ("ls");print "Hello\n"'
Statement unlikely to be reached at -e line 1.
        (Maybe you meant system() when you said exec()?)
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citromatik