#! perl -w

use strict;
package MyModule;

sub isFile {
    my $file = shift; 	# a file in local directory expected	
    my $verbose = shift; 	# optional	
    if (! -f $file) {
        print STDERR "*** no such file $file in current directory\n";
        return 0;
    }
    else {
        print STDERR "... isFile $file\n" if $verbose;
        return 1;
    }
}

# more subs ...

1;

##</code><code>##

#! perl -w
use strict;

use MyModule;

# before change to OO and after

MyModule::isFile("MyModuleTest.pl");
MyModule::isFile("NotHere");

##</code><code>##

#! perl -w

use strict;
package MyModule;

sub new {
    my ($class, %args) = @_;
    my $self = {
        class           => $class,
        verbose         => $args{verbose} || 0,
        otheropt        => $args{otheropt} || 0,
    };
    bless $self, $class;
    return $self;
}

sub isFile {
    my $self = shift if ref $_[0] && ref $_[0] eq $_[0]->{class};
    my $file = shift;   # a file in current directory expected  
    if (! -f $file) {
        print STDERR "*** $file: file not found in current directory\n";
        return 0;
    }
    else {
        print STDERR "... $file: file found in current directory\n" if $self->{verbose};
        return 1;
    }
}

# more subs ...

1;

##</code><code>##

#! perl -w
use strict;

use MyModule;

# works before change to OO and after

MyModule::isFile("MyModuleTest.pl");
MyModule::isFile("NotHere");

# works after change to OO

my $mod = MyModule->new;

$mod->isFile("MyModuleTest.pl");
$mod->isFile("NotHere");

$mod = MyModule->new( verbose => 1 );

$mod->isFile("MyModuleTest.pl");
$mod->isFile("NotHere");