use strict;

my $keyword = "START";
my $length  = 20;

my $file = <<EOF;
something START text1
 text2 ........................ text 3
text 4 START more text
still more text
EOF

$file =~ s/(?:\s+|\n+)//gc;

my @hits = $file =~ /$keyword(.{$length})/g;

for (@hits) {
  print $_."\n";
}