You could do this:

#!/bin/perl -w
my( $_var, $_val)= split /\s*=\s*/, $ARGV[0];
die "can't use $_var as a variable name" if( $_var=~ /^_va[rl]$/);
${$_var}= $_val;
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This is higly unsafe though, you cannot use strict (at least not strict 'refs') and you risk overwriting one of you existing variables.

Why not just use a hash:

#!/bin/perl -w
use strict;
my( $_var, $_val)= split /\s*=\s*/, $ARGV[0];
my %hash=( $var => $val);
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If you call both snippets with foo=bar as argument The first one lets you use $foo and the second one $hash{foo}.