use warnings;
use strict;

my $log_file  = 'file.log';
my $uid       = 'xy1234';
my $threshold = 20;
my @occurrences;

open my $fh_log, '<', $log_file or die "opening $log_file: $!";

LINE:
while (defined(my $line = <$fh_log>)) {
  next LINE unless $line =~ m{ uid=$uid }xms;
  push @occurrences, $line;  
  }

close $fh_log or die "closing $log_file: $!";

if (@occurrences > $threshold) {
  print "occurrences for $uid above $threshold \n";
  print @occurrences;
  }