want to define the value of a var on the commandline is this possible ?

It certainly is. See the -s option in the perlrun manpage. Here's a brief example

shell> cat > dash_s_switch.pl
print "\$foo is $foo\n";
^D
shell> perl -s dash_s_switch.pl -foo="a string"
$foo is a string
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HTH

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broquaint