i solved the problem by using

open (FH_SECOND,"<",\$second_file) or die $!; 

but can i use the declaration $line_second=<FH_SECOND>; to save the a 
+line in the variable line_second
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Get rid of the backslash:

open (FH_SECOND, "<", $second_file) or die $!;
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Now, line_second should contain the first line of your file, which you can confirm by printing it.