For this to work, you need new to return an object. Right now it just returns undef.

Firstly, you need to read perlsub to learn how subs work, and the important difference between $_ and @_.

Then turn to perlootut to learn the basics of object-oriented programming in Perl.

Here's how you could write your code...

use strict;
use warnings;

{
    package testor;
    
    sub new {
        my $class = shift;
        return bless {
            component_name => $_[0],
        }, $class;
    }
    
    sub component_name {
        my $self = shift;
        return $self->{component_name};
    }
    
    sub isExist {
        my $self = shift;
        return 0;
    }
}

my $myComponent = '7-Zip 9.20 (x64 edition)';
my $checker = testor->new($myComponent);

if( $checker->isExist ) {
    print $checker->component_name, " already installed\n";
}
else {
    print $checker->component_name, " not installed\n";
}
[download]

But personally I'd write it as...

use strict;
use warnings;

{
    package testor;
    use Moo;
    
    has component_name => ( is => 'ro' );
    
    sub isExist {
        my $self = shift;
        return 0;
    }
}

my $myComponent = '7-Zip 9.20 (x64 edition)';
my $checker = testor->new(component_name => $myComponent);

if( $checker->isExist ) {
    print $checker->component_name, " already installed\n";
}
else {
    print $checker->component_name, " not installed\n";
}
[download]

Either way, read those bits of Perl documentation I mentioned, and learn why your original attempt is wrong.

It's very kind of you. Thanks a lot!