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

my $in_filename = shift @ARGV;
my @extra_parms = @ARGV;

sub usage
{
   print "Usage:\n Searches for selected 'sentences' in input file\n";
   print " A sentence ends with a period(.) or could be the\n";
   print " end of line '\\n'\n";
   # other text to expain what "rules are goes here....
   # give an example of the command.
   print " Example: mysearch infilename\n\n";
   print " The program will prompt user for the search terms.\n";
   
   exit(1); #this is an error exit (non-zero return value)
}

# The very basic "sanity" checks to display the usage() message.

if (!defined($in_filename) or @extra_parms>0
    or $in_filename =~ /^(-)?\?/ or $in_filename =~ /^\s*-(-)?h(elp)?/i)
{
    usage();
}

if (! -e $in_filename)
{
   print "Error! input file name: $in_filename does not exist\n\n";
   usage();
}

print "Enter search parameter(s), one per line or end\n";

my $input;
my @search_tokens;

while ( (print "search for: "), $input=<STDIN>, $input !~ /^\s*END\s*$/i)
{

    next if $input =~ /^\s*$/; # re-prompt on a blank input line
    $input =~ s/^\s*//;        # delete leading spaces
    $input =~ s/\s*$//;        # delete trailing spaces
 
    if ($input =~ /^\S+\s+\S+/)
    {
       print "Error! Only one search term per input line!\n";
       next;  
    }

    # Note: here tr// counts the number of characters which are not
    # in the set, without modifying the $input variable.
    
    if ($input =~ tr/A-Za-z0-9_//c) # must be legal characters
                                    # for a filename!
    {
       print "Error! Illegal character! only A-za-z0-9_ allowed!\n";
       next;
    }
    
    if ($input =~ /^\d/) # must be legal characters for a filename!
    {
       print "Error! Token cannot start with a number!\n";
       next;
    }
    
    push (@search_tokens, $input);
}

if (@search_tokens==0)
{
   print "Error! No search tokens entered..exiting..\n";
   exit (2);    # another error exit with error code 2
}

# For debugging, dump the tokens back out

print "\n"; #just a space line
print "Search Terms are:\n";

foreach my $token (@search_tokens)
{
   print "token=\'$token\'\n";
}

__END__

Get the above working and tested, then move on the next section
of the program. You can make a separate test program that just
Hard codes the $filename and @search_tokens. Get that
working, then move on to another step.