use warnings; use strict;
my %global;
try($_) for 0,0,0,1,0;

sub try {
    my $xyz = shift;
    if($xyz) { $global{globalvar} = undef; }
    printf "try(%d) => globalvar %s\n", $xyz, exists $global{globalvar} ? "exists" : "doesn't exist";
}
__END__
try(0) => globalvar doesn't exist
try(0) => globalvar doesn't exist
try(0) => globalvar doesn't exist
try(1) => globalvar exists
try(0) => globalvar exists