if ( not defined $ENV{'foo'} ) {
    # foo being some variable already set by your shell
    # script rather than a separate flag

    open ( my $se, '<', './set_env.sh' )
        or die "Can't read ./set_env.sh : $!\n";
    while ( <$se> ) {
        next unless /(\w+)=["']?(\w+)["']?/;
        # handling exotic formatting left as an exercise
        $ENV{$1} = $2;
    }
}

print "foo is $ENV{'foo'}!\n";