If you really want to pass your hash around

login($username, %logins);

then the start of the login sub should be

my $username = shift;
my %logins = @_;
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and this

if ($pass <=> $logins{$username})
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becomes

if( $pass eq $logins{$username} )
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ie use a string comparison operator.
http://perldoc.perl.org/perlop.html#Equality-Operators

Numeric test:

if ($selection == "1")
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becomes

if ($selection == 1)
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Cheers
Chris