There is not a single command to insert a line into a file. The Tie::File module gives a convenient way to do it.

use Tie::File;
my $newline = "whatever";
{
    tie my @array, 'Tie::File', '/path/to/file.txt';
    unshift @array, $newline;
}
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If you wish to insert a line elsewhere, splice works with that.

After Compline,
Zaxo