For fun, a more verbose version that you might find easier to understand or maintain: 

# print-first.pl. See Perl Monks [id://11154123].
use strict;
use warnings;

my $fname = shift or die "usage: $0 file\n";
open( my $fh, '<', $fname ) or die "error: open '$fname': $!";
my %hash;
my $line;
while ( defined($line = <$fh>) ) {
   chomp $line;
   $line =~ s/^\s+//;                        # remove leading
   $line =~ s/\s+$//;                        # and trailing whitespace
   next unless length $line;                 # ignore empty lines
   next if $line =~ /^#/;                    # ignore comment lines
   next unless $line =~ /^(\w+)\s+(\d+)$/;   # ignore lines that do no
+t match
   my $key = $1;
   my $val = $2;
   next if exists $hash{$key};               # ignore if already seen
   ++$hash{$key};
   print "$key $val\n";
}
close $fh;

[download]

Example run: 

$ cat t1.txt
nick 5
nick 10
nick 20
john 78
erik 9
erik 12

[download]

$ perl print-first.pl
usage: print-first.pl file

$ perl print-first.pl t1.txt
nick 5
john 78
erik 9

[download]

See Also

Updated: Added "See Also" section

In reply to Re: How can I keep the first occurrence from duplicated strings? by eyepopslikeamosquito
in thread How can I keep the first occurrence from duplicated strings? by Anonymous Monk

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