Yeah, bad idea.
Consider the two proposed automatic mys in the following code. It avoids an obvious compile time fault ($line), while causing a silent run time fault ($indux).
use strict;
use warnings;
my $index = 0;
while ($line = <>)
{
frobnicate($indux++, $line);
}
Appended Re: LanX's comment
If only applying the my to assignments initializations, then consider this code with the same issues:
use strict;
use warnings;
my $faultsInARow = 0;
while ($line = <>)
{
if (frobnicate($line))
{
$faultsInaRow = 0; #Initializing a new variable by mistake.
}else{
$faultsInARow ++;
}
}