In your original code, $f was a variable holding the whole code of the module as a string. So of course

$f->new
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would produce syntax errors! You need to replace $f with whatever name you gave the module, i.e., the name you used in the package declaration.

Here is an example to show how this works:

(1) File “Widget.pm”:

#! perl
use strict;
use warnings;
package Widget;

sub new
{
    my ($class, $name) = @_;
    my  %self = (name => $name);
    bless \%self, $class;
}

sub say_hello
{
    my ($self) = @_;
    print "$self->{name} says \"Hello!\"\n";
}

1;
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(2) The main .pl script (in the same directory as Widget.pm):

#! perl
use strict;
use warnings;
use autodie;

open(my $fh, '<', 'Widget.pm');
my $f = do { local $/; <$fh> };
close $fh;

eval $f;
warn $@ if $@;

my $widget = Widget->new('Gromit');
$widget->say_hello();
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When I run this, I get:

Gromit says "Hello!"
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as expected.