#! perl -sw
use strict;

my $term = shift;

$/ = 'Header';

while( <> ) {
    chomp;
    print 'Header', $_ if 1+index $_, $term;
}

__END__
[11:28:45.37] c:\test>junk34 lima 886391.dat >junk.dat

[11:28:57.83] c:\test>