You have opened a file for reading input but, you are trying to write into it.

Filehandle FH opened only for input at pl.pl line 18, <FH> line 5

Open the file in read/write mode as below,

#!/bin/perl
use strict;
use warnings;
use Data::Dumper;
my $name="mouse";
pen (FH, "+<","datafile.txt")||die "cannot open file";
my (%HoA,$venue);
while ( <FH> ) {
   next unless s/^(.*?):\s*//;
   $HoA{$1} = [ split /,/ ];
 }
print Dumper(%HoA);
my $success="yes";
my $description="new string";
push @{ $HoA{$name} }, $success, $description;
for $venue ( keys %HoA ) {
   print FH "$venue @{ $HoA{$venue} }\n";
}
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Update:

++Athanasius,Missed use strict; part.

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