So mainarr will be having three arrays.

No:

use strict;
use Data::Dumper;

my @mainarr;
my @arr1=qw(a b c d e);
my @arr2=qw(f g h i j);
my @arr3=qw(k l m n o);
push(@mainarr,@arr1);
push(@mainarr,@arr2);
push(@mainarr,@arr3);

print Dumper \@arr1;
print Dumper \@arr2;
print Dumper \@arr3;
print "-- mainarr\n";
print Dumper \@mainarr;
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If you want to keep the structure of @arr1 etc., you will have to store references to the arrays in @mainarr:

...

push(@mainarr,\@arr1);
push(@mainarr,\@arr2);
push(@mainarr,\@arr3);

print Dumper \@mainarr;
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Then you can get at the values in @arr2 via $mainarr->[1]. Also see References Quick Reference.