grep -il "^$input" *
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is equivalent to the following Perl one-liner in bash:

perl -lne '$seen{$ARGV} = 1 if /^'"$input/i }{ print for keys %seen" *
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or the following script:

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

my $input = shift;
for my $file (glob '*') {
    open my $IN, '<', $file or die $!;
    while (<$IN>) {
        if (/^$input/i) {
            print "$file\n";
            last;
        }
    }
}
[download]

Run as script.pl "$input".