use strict;
use warnings;

my $new_shell = "/bin/nologin";

# in-place editing with backup
$^I = ".bak";

# diamond operator "<>" reads from command line arguments or STDIN
while ( my $line = <> ) {

    # skip empty lines
    next if $line =~ /^\s*$/;

    # split on ":" and return the 7th field (list slice)
    my $old_shell = ( split ":", $line )[6];

    $line =~ s/$old_shell/$new_shell/;
    print "$line\n";
}