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die "usage: perl $0 file_nm line_nb col_start col_end replacement \n" 
+unless @ARGV;
die "Invalid number of arguments \nusage: perl $0 file_nm line_nb col_
+start col_end replacement \n" if @ARGV ne 5;
...
chomp ($replacement=`perl -ne "substr($content, $col_start, $col_end, 
+$string);print $content"`); #get the replaced line
#print "New content \n$content \n";
`perl -pi -e "s/$content/$replacement/" $file_nm`; #now substitute the
+ whole line with the new replaced line

use File::Basename;
die "usage: perl $0 file_nm line_nb pos_start-pos_end replacement \n" 
+unless @ARGV;
...

close(WRITE_HN);
close(READ_HN);