Perhaps you should have a look at the source of File::Spec. Or perhaps not. Think about the following construct first:

(Each code block represents a properly named file)

package MyProject::OSSpecific::base;

use strict;
use warnings;

sub niceOSName
{
    my $class=shift;
    die "$class does not implement an OS specific niceOSName() method"
+;
}

sub tempDir
{
    die "I don't know how to find a temp directory";
}

sub pathSep
{
    return '/';
}

sub installTo
{
    die "Should not happen";
}

# much more methods

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package MyProject::OSSpecific::doslike;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::base';

sub tempDir
{
    return 'c:\\temp';
}

sub pathSep
{
    return '\\';
}

# much more methods

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package MyProject::OSSpecific::dos;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::doslike';
use Some::DOS::Specific::Module;

sub niceOSName
{
    my $x=`command /c ver`;
    chomp $x;
    return $x;
}

sub installTo
{
    return 'C:\myproject';
}

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package MyProject::OSSpecific::MSWin32;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::doslike';
use Win32;

sub niceOSName
{
    return Win32::GetOSName();
}

sub installTo
{
    return 'C:\\Program Files\\MyProject';
}

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package MyProject::OSSpecific::unixlike;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::base';

sub niceOSName
{
    my $x=`uname -a`;
    chomp $x;
    return $x;
}

sub tempDir
{
    return '/tmp';
}

sub pathSep
{
    return '/';
}

# much more methods

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package MyProject::OSSpecific::freebsd;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::unixlike';
use Something::Created::Only::For::FreeBSD;

sub installTo
{
    return '/usr/local/myproject';
}

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package MyProject::OSSpecific::linux;

use strict;
use warnings;
use parent 'MyProject::OSSpecific::unixlike';
use Something::Very::Linux::Specific;

sub installTo
{
    return '/opt/myproject';
}

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packame MyProject::OSSpecific;

use strict;
use warnings;

use parent 'MyProject::OSSpecific::'.$^O; # <-- this is the most impor
+tant trick here.

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use strict;
use warnings;

use MyProject::OSSpecific;

print "This program should be installed to ",MyProject::OSSpecific->in
+stallTo(),"\n";
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Alexander