Try this (more idiomatic) version:

 #!/usr/bin/perl
use strict;
use warnings;

my ($fname, $dupcount) = @ARGV;

die "USAGE: extract_duplicate_rows <input file> <# of duplicate rows>\
+n\n"
  unless $fname and  $dupcount;

open(my $IFILE, "<", $fname) or die "Cannot open $fname: $! ";
my @buf;

while (my $line = <$IFILE>)
{
    if (@buf){
       if (substr($line,0,10) eq substr($buf[0],0,10)){
        push @buf,$line;
        next
       };
       if (@buf ==$dupcount){
        print @buf;
       };
     };
    @buf=($line);
}
@buf==$dupcount and print @buf;
close $IFILE;
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This assumes that the input file is sorted.