# start your program with these. trust me
use strict;
use warnings;


# hash table of search strings and insertion code
my %insertions = (
    'search string' => 'code insertion',
    foo => 'bar',
    alpha => 'beta',
);


# read in array of filenames from a directory
opendir (my $DH, $directory);
my @files = $readdir $DH;
closedir $DH;


# iterate over array of filenames
for my $filename (@files){
    
    # process your file here
    # or use a subroutine
    subName($filename);
}


# basic subroutine
sub subName{

    # get first arg passed
    my $filename = shift;

    # if you need a return value
    return $foo;
}


# opening files. $TFH = target filehandle
open (my $TFH, ">", $targetfile);
open (my $SFH, "<", $sourcefile);

# read source file line by line
while (my $line = <$SFH>){ 
    print $TFH, $line;

    # use regular expression to find matching string 
    if ($line =~ /$matchString/){   
        print $TFH, $snippet;
    }
}
close $TFH;
close $SFH;