The defined or assignment operator only assigns the right hand value if the value to the left is undefined so you can use that to assign just the first value found to a hash:

use strict;
use warnings;

my $input = <<IN;
nick 5
nick 10
nick 20
john 78
erik 9
erik 12
IN
my %hits;

open my $fin, '<', \$input;

while (my $line = <$fin>) {
    next if $line !~ /^(\w+)\s+(\d+)/;
    $hits{$1} //= $2;
}

print "$_: $hits{$_}\n" for sort keys %hits;
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Prints:

erik: 9
john: 78
nick: 5
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