You should get into the habit of using lexical variables instead of package variables, so:

$num = <STDIN>;
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Would become:

my $num = <STDIN>;
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While perl supports C style for loops it is usually preferred to use perl's for loops, so:

for ( $x = 1; $x <= $y; $x++ )
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Would become:

for my $x ( 1 .. $y )
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