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#Open the 'txt' file for reading
open FH, '<', "$file_name.txt" or die "Error:$!\n";

#Open the 'txt' file for writing. Creates the 
#file_name if it doesn't already exist 
#and will delete/overwrite a pre-existing file of the same name
open FH, '>', "$file_name.txt" or die "Error:$!\n";

#Open the 'txt' file for appending. Creates the 
#file_name if it doesn't already exist
open FH, '>>', "$file_name.txt" or die "Error:$!\n";

#Open the 'txt' file for a 'read/write'. 
#Will not create the file if it doesn't 
#already exist and will not delete/overwrite
#a pre-existing file of the same name
open FH, '+<', "$file_name.txt" or die "Error:$!\n";

#Open the 'txt' file for a 'read/write'. Will create 
#the file if it doesn't already exist and will 
#delete/overwrite a pre-existing file
#of the same name
open FH, '+>', "$file_name.txt" or die "Error:$!\n";

#Open the 'txt' file for a 'read/append'. Will create 
#the file if it doesn't already exist and will 
#not delete/overwrite a pre-existing file
#of the same name
open FH, '+>>', "$file_name.txt" or die "Error:$!\n";

close FH or die "Error in closing the file ", __FILE__, " $!\n";